1. Field of the Invention
The present invention relates to wireless terminal devices and, more particularly to a wireless terminal device, such as a portable telephone, having a receiving unit with a direct conversion system in which a received signal is converted directly into a base band signal.
2. Description of the Background Art
In a heterodyne conversion system for receivers, which has been well known, a received signal is converted into an intermediate frequency (IF) signal by a mixer (MIX) and then an unwanted wave is removed by a band-pass filter (BPF) for the IF. After that, complex envelope detection is performed by a quadrature mixer (QMIX) to supply a base band signal. In the heterodyne conversion system, however, as the intermediate frequency exists an intermediate frequency circuit, such as a band-pass filter, must be provided, resulting in the increase in the number of components and space for mounting the components.
On the other hand, in a direct conversion system, a received signal is directly converted into a base band signal by a quadrature mixer. FIG. 1 is a block diagram referenced for describing the direct conversion system.
FIG. 1 shows an example in which the direct conversion system is employed for a portable telephone which operates according to a certain communication system, such as CDMA system, in which transmission and reception as performed simultaneously and, a transmission system connected to a duplexer (DUP) 2 is not shown. In FIG. 1, a high-frequency signal received by an antenna 1 is supplied to a low-noise amplifier (AMP) 3 via duplexer 2, a received signal of a low level is amplified, an unwanted high-frequency band component such as an external interference wave is removed by a band-pass filter (BPF) 4 and the remaining component is converted into a base band signal by a quadrature mixer (QMIX) 5.
Here, the relation between a frequency frx of the received signal, which is an output from band-pass filter 4 and a local oscillation frequency flo of quadrature mixer 5 is represented by the following expression:fbb=frx−flo,where fbb is the base band signal. As the frequency of the base band signal is sufficiently lower than that of the received signal, frx=flo holds.
The base band signal obtained through conversion is amplified by a low-noise amplifier (AMP) 6 and the resulting signal is supplied to a low-pass filter (LPF) 7 where an interference wave such as an adjacent channel component is removed from the base band signal.
Further, the base band signal is supplied to a variable gain amplifier (VGA) 8 and amplified to maintain an input to an A/D converter 9 in the next stage at a constant level and, the resulting signal is converted into a digital signal by A/D converter 9 and, demodulated at a demodulation circuit (DEM) 10. To achieve this operation, the level of the signal supplied to A/D converter 9 must not become saturated at A/D converter 9 or exceed a level of resolution.
In the receiver of the direct conversion system shown in FIG. 1, the elimination of the mixer and the band-pass filter for the intermediate frequency required in the heterodyne conversion system is allowed to achieve a simple circuit structure. In addition, a spurious response of the receiver to a signal represented by an image signal can be mitigated whereby the high-frequency band-pass filter can be replaced with a smaller band-pass filter 4.
In the portable telephone, however, the level of the base band signal supplied to A/D converter 9 at the onset of the operation, such as at power-on, is not predictable. To deal with this problem, the gain of variable gain amplifier 8 can be set to a suitable value based on an assumption.
When the initial set value of the gain of variable gain amplifier 8 is high, however, if a signal of a higher level than an input voltage range of A/D converter 9 as shown in FIG. 2A is supplied and is amplified with a high amplification factor by variable gain amplifier 8 and input to A/D converter 9, A/D converter 9 is saturated and the detection of the signal becomes impossible. In this case, a suitable level of the gain of variable gain amplifier 8 to be set for the next step cannot be known.
Conversely, when the initial set value of the gain of variable gain amplifier 8 is low, if a signal of a lower level than the resolution of A/D converter 9 as shown in FIG. 2B is supplied, the signal is hardly amplified by variable gain amplifier 8 and, the signal detection is not possible with the resolution of A/D converter 9. Hence, a suitable level of the gain of variable gain amplifier 8 to be set for the next step cannot be known.
Therefore, it is necessary to predict the suitable gain of variable gain amplifier 8, and to change the setting of the value a few times until the suitable value is set. Thus, the setting of the gain of variable gain amplifier 8 to an optimum level takes time.
To solve the above-described problem, the input voltage range of A/D converter 9 may be widened and A/D converter 9 with fine resolution may be employed. Such A/D converters, however, consume large current and are large in circuit size and their use in portable telephones is subjected to a certain limitation.
Thus, a main object of the present invention is to provide a wireless terminal device allowing a swift and secure gain control of the variable gain amplifier at the start of the operation.